Automatic volume control circuit



l1g- 7, 1951 o. L. MacsoRLEY AUTOMATIC' VOLUME CONTROL CIRCUIT f Filed May 24, 1947 ATTORNEY Patented Aug. 7, 1.*951

UTOMATIC VOLME CONTROL CIRCUIT lin L. MacSorley, Collingswood, N. J., assignor to Radio Corporation America, a ,corporation .of Delaware Application V'May 24, 1947, Serial No. 750,208

l 2 Claims.

' My present invention relates generally to auitomatic volume control .circuits for radio receivers, and more particularly to ,an limproved arrangement for protecting the automatic volume 'control (AVC) circuit of `a lreceiving 4system sub- 2 ject to strong signal reception.

' `itis Aorten desirable in' a radio receiver to `use a large vantenna gain to' improve the signal to noise ratio. In `receivers equipped with AVC circuits there arises `situations where the presence of a strong radiore'quency `sig-nal may zlead to the production of aradio frequency signal voltage at .the control .grid orf'fthe iirst tube havingxa greater peak I'amplitude than the AVIC voltage at the grid o the same tube. For example, radio frequency signal voltages of the order of several volts may occur `at the :antenna loi :an aircraft receiver when the plane is flying closeto another plane, or when 'it is "in close proximity 4vto the transmitter at the airlield 4control tower. Normally in a 'receiver h-aving 7a `large gain ahead .of the rst tube control `grid (lo to '20 times), the AVC voltage applied to the grid of .the lirst tube will cause the gain through'that tube to change to a loss for large inputs, and the actual radio frequency voltage :at'the :grid of the second tube will beless than' that at the control grid of the Yfirst tube.

' Where, as ystated above, the development of a strong signal at the control grid y,of the first tube attains a greater peak .amplitude than the AVC voltage 4at that grid, grid reotiiication will take place. As a 'resulta "negative direct current Voltage will be produced Lat that grid' which is `greater than the AV C bias applied over the AVC circuit. Since the .control grid of the first tube is connected to the common AVC line all the tubes that 'normally receive AVC voltage will have applied thereto this more negative voltage. Hence, the gain of the .controlled tubes is reduced below that which :is required to keep a constant 1output,:andthere will resulta .rapid decrease in output terminating in vcomplete cutoff of the receiver.

' There are several ways Sto deal with this problem. The most yobvious is not to apply any AVC voltage to the first tube. However, this is not satisfactory, because under normal operating conditions `it is desirable to .apply AVC voltage to the rst tube. There are other expedients which only partially solve the problem.

In accordance with v my present invention, l .provide a .means 'of preventing the :blocking oi the receiver by virtue of high negative voltage on the AVCfcircuit, but without preventing AViC rac- 2 tion on the first tube during normal operating conditions.

It is an object of my invention to provide a uni-directional conductive device in the AVC line between the control rgrid of the rst controlled tube and the control grid of each of the following tubes, the device being conductive when .the AVC line is more negative than the control grid of the first tube .and 'being non-conductive when the control grid is more negative than the AVC line.

Specincally, in accordance with my invention the control grid of the rst tube of a radio receiver provided with AVC, 'is connected to the AVC line through a coupling lmeans which will provide a direct coupling between the AVC line and the control grid of the lirst tube as long as the amplitude of the negative voltageon the AVC line exceeds. the peak amplitude of the signal applied to the ycontrol grid, but will break vthis connection whenever the peak amplitude of the signal applied Vto the control grid becomes sufiicient-ly large to produce by grid rectification a negative direct xctn-rent voltage on the grid greater than thatapplied to the AVC line by the usual AVC rectiii'er.

-Still 4other features and objects of 'my invention will-best be understood by reference to the following description taken in connection with the drawing, in which l have indicated diagrammatically a circuit organization whereby myinvention may be carried into eiect.

The receiving system shown in the drawing is schematic in representation, since those skilled fin the tart are fully fav/are -iof the manner -of constructing ra radio receiving system employing AVC, whether it be utilized 4in connection with vamplitude modulated carrier waves, vfrequency modulated carrier waves or leven phase modulated carrier waves. Further it fis to be under- `:stood thatthe receiving system is of the type employed in connection with aircraft communication. Specifically, the Vreceiver* schematically shown in the drawing is assumed to representan .aircraft receiver.

r-l`fhe numeral I denotesuthe first radio fre'- @Uuency amplifier tube, and numfreral :32 designates the second radioirequencgy 'amplier tube. While I have shown these tubes as penftodes, it is to be lclearly understood that they may `be of any other vsuitable type; The control or .input 'gridY 3 of amplifier tube lI ,is shown Icoupled by a .rad-io fre'- fquency'oo-upli-ng lcondenser to the high `alternating potential sid'e of the selective resonant in.- pnt lcircuit fwhosei oppositesilii eIOHIJed.

The numeral I designates any suitable frequency selecting means, as, for example, the usual variable capacity device. The primary circuit 1 is to be understood as being connected in the antenna circuit, whether it be a dipole or grounded antenna.

The control grid 3 is returned to the grounded end of a suitably bypassed grid bias resistor 8 over a pair of series-arranged resistors 9 and Ic. The junction point II of resistors 9 and Il] is connected to the AVC line i2 through a device of unidirectional conductivity. Specifically, I have shown the uni-directional device as a diode i3 whose anode I 4 is connected by lead I5 to junction point I I, a radio frequency bypass condenser I6 connecting anode I4 directly to ground. In other words the diode plate I 4 is bypassed to ground for all radio frequency signals, while the diode cathode Il is connected to the AVC lead I2.

The coupling network between the plate I3 of tube I and the control grid I9 of amplier tube 2 is represented as being included in the boX 20. It is to beunderstood that the dotted line resonant circuit 2i may be any suitable form of selective resonant circuit. This circuit may include a frequency selective device which is operated concurrently wth the device 6 so as to tune circuits 5 and 2| to a common frequency. The control grid I9 of amplifier tube 2 is connected to the AVC voltage source through series-arranged resisters 22 and 23 whose junction point 24 is bypassed to ground by condenser 25 for all radio frequency signals. The cathode of amplifier tube 2 is connected to ground through a suitably bypassed bias resistor 3Q. Of course, the selective amplified output of tube I is applied through coupling condenser 3I to the control grid I9 of amplifier tube 2.

The box 40 schematically represents additional stages of selective carrier amplification. However, they may include the usual converter stage and intermediate frequency amplifier stages, where the receiver is of the superheterodyne type. Whether the signals received are ampli tude modulated or frequency modulated, the receiving system so far as AVC action is concerned will function in substantially the same manner. That is to say, some form of demodulating or detector device 4I will be employed to develop across a load resistor 42 the direct current voltage used for AVC action. The numeral 4I designates a box which schematically represents the detector.

Assuming that the detector is of the diode type,

Aload resistor 42 will be connectedr in circuit with the diode so as to develop the modulation and direct current voltage components of the rectied carrier energy. Condenser 43 bypasses load resistor 42 for carrier frequency. The ungrounded end of resistor 42 will vary in a negative potential sense relative to ground, and the amplitude of the negative voltage will be proportional to the intensity of the carrier amplitude. AVC lead I2 is connected to the negative end of resistor 42 through filter 44 which functions to suppress all pulsating voltage components in the AVC Voltage. The time constant of network 44 is chosen to permit AVC line I2 to act in known manner. The condenser 5i] is included in the path which derives the modulation voltage component from across load resistor 42. Specifically, condenser 50 will be connected to the audio frequency amplifier network of the receiver.

The coupling device I3 between lead I5 and AVC lead I2 could just as well be a crystal rectifier, such as one of the type 1N34, whose anode is connected through an isolating resistor to the control grid 3, and a second resistor II) going from the anode I4 to ground. As long as the AVC lead I2 is more negative than the potential of anode I4 the diode will be conductive, because in that case the cathode I1 is negative with respect to the diode anode I4. Hence, a negative voltage equal to the AVC voltage less the drop across the rectifier I3 will be applied to the grid 3 of amplifier tube I. If the radio frequency signal at grid 3 becomes sufficiently large to produce, by means of grid rectication, a negative voltage at the anode I4 which is larger than the AVC voltage, then the diode I 3 will become non-conductive. This follows from the fact that the anode I4 will be negative with respect to its cathode, and, hence, the diode I3 will not conduct. This will prevent the negative voltage produced by grid rectification at tube I from being applied through resistors 22 and 23 to the amplifier control grid I9, and through corresponding resistors to any other of the selective amplifier grids to which AVC is normally applied.

It might be thought that a strong signal could cause the second tube grid I9 to swing sufficiently positive to cause grid rectification in its grid circuit thereby to cause diode cathode I 1 to assume a negative potential relative to anode I4, and thus prevent diode I3 from breaking down. This is not so, because normally in a receiver having large gain ahead of the grid of the first tube (10 to 20 times) the AVC bias on the rst grid will cause the gain through the first tube to change to a loss for large inputs. The actual radio frequency voltage at the second tube grid, therefore, will be less than that at the first tube control grid. Should, in a special case, the second tube grid draw grid current, then an additional diode can be inserted between grid I9 and AVC line I2.

In normal receiver operation the negative AVC voltage keeps the cathode I 'I negative relative to anode I4, and diode I3 is conductive thereby permitting normal AVC operation of the receiver. If, now, a strong radio signal, as of the order of several volts when an aircraft antenna is close to another plane or tower transmitter, causes a peak radio frequency signal at grid 3 which is greater than the sum of the AVC and cathode voltages, grid 3 will rectify and provide direct current voltage across resistors 9, I0. As soon as the anode I4 becomes more negative than AVC line I2, the diode I3 becomes nonconductive which isolates grid 3 from the remainder of the AVC circuit until conditions return to normal.

While this present system has been described as being used with the rst tube of a receiver, its application is not limited to this use. The present invention may be used at any point in an amplifier circuit where there is danger of the radio frequency input signal increasing faster than can be taken care of by the AVC circuit. That is, the tube I may be preceded by one or more amplifiers which may or may not be controlled by AVC, and which prior amplier is capable of producing the excessive gain.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described. but

that many modifications may be made without departing from the scope of my invention.

WhatIclairn is:

1. The combination, in a radio signal receiver, of a plurality of signal amplioation stages connected in cascade relation to each other, means for producing a direct current control potential proportional to signal current amplitude includ* ing a single direct current control path, circuit means connecting said direct current control :path to each of said amplication stages whereby the gain of said amplification stages is varied in inverse relation to the amplitude of said signal current, and a uni-directional conducting device connected serially in said single direct current control path between said circuit means connecting the first of said amplicaticn stages and the second of said amplification stages to the said direct current control path for eiectively opening said control path upon signal overload of the first of said amplication stages whereby eX- cessive control voltages developed by the first of said amplification stages are not applied to succeeding ampliication stages.

v 2. In a signal amplifier system having a plurality of signal amplification stages connected in cascade relation to each other, each of said stages including an input grid, the combination of a control Voltage circuit comprising, means for producing a direct current control potential proportional to signal current amplitude, filter means connected to said control voltage produc-V ing means whereby fluctuations appearing in said control voltage are reduced, individual means including additional iilter means connecting said first named iilter means to the input grids of the second and succeeding stages whereby said filtered direct current control potential is applied to said grids for varying the gain of said amplification stages in inverse relation to the amplitude of said signal current, circuit means connected to the input grid of the first stage, and a uni-directional conducting device serially connected between said rst illter means and said circuit mean-s, said uni-directional conducting device being so poled as to be non-conductive when said filtered direct current control voltage is positive with respect to voltage developed in the last named circuit means due to grid rectication.

OLIN L. MACSORLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,073,486 Koch Mar. 9, 1937 2,144,304 Braden Jan. 17, 1939 2,199,350 Shofstall Apr. 30, 1940 

